In a liquid crystal display device, an active matrix liquid crystal display device, in which pixel electrodes are provided in matrix and transistors are used as switching elements connected to respective pixel electrodes in order to obtain an image with high quality, has attracted attention.
An active matrix liquid crystal display device, in which transistors formed using a metal oxide for a channel formation region are used as switching elements connected to respective pixel electrode, has already been known (see Patent Document 1 and Patent Document 2).
It is known that an active matrix liquid crystal display device is classified into two major types: transmissive type and reflective type.
In the transmissive liquid crystal display device, a backlight such as a cold cathode fluorescent lamp or the like is used and optical modulation action of liquid crystals is utilized to choose one between the two states: a state in which light from the backlight passes through liquid crystal to be output to the outside of the liquid crystal display device and a state in which light is not output, whereby bright and dark images are displayed; further, image display is performed in combination of these.
Since the backlight is utilized in the transmissive liquid crystal display device, it is difficult to recognize display in the environment with strong external light, for example, outdoors.
In the reflective liquid crystal display device, the optical modulation action of liquid crystals is utilized to choose
one between the two states: a state in which external light, that is, incident light is reflected by a pixel electrode to be output to the outside of the device and a state in which incident light is not output to the outside of the device, whereby bright and dark images are displayed; further, image display is performed in combination of these.
Compared to the transmissive liquid crystal display device, the reflective liquid crystal display device has the advantage of low power consumption since the backlight is not used; therefore, a demand for the reflective liquid crystal display device as a portable information terminal has increased.
Since external light is utilized in the reflective liquid crystal display device, the reflective liquid crystal display device is suited for image display in the environment with strong external light, for example, outdoors. On the other hand, it is difficult to recognize display when the liquid crystal display device is used in a dim environment, that is, in the environment with weak external light.